Cross-pollinator considerations

How to think about cross-pollination when planning orchards. By Anna Mouton.

Effective cross-pollination is crucial for maximising productivity of self-incompatible deciduous-fruit orchards over their lifetime. This article shares insights on cross-pollinator selection and management from horticultural adviser and plant breeder Dr Nigel Cook of Innovapome.

Choosing the cultivar

Apples and most stone fruit need cross-pollination to set fruit. Pears can set unfertilised fruit with a little hormonal help from plant growth regulators but will produce better yields and fruit quality when cross-pollinated.

Peaches and nectarines are self-compatible and do not need cross-pollinators.

Cross-pollination requires genetic compatibility — read a full explanation of compatibility elsewhere in this issue. The short version is that cultivars sharing two S-alleles are incompatible, and cultivars sharing one are partially compatible. Fully compatible cross-pollinators are preferable to partially compatible cross-pollinators.

Strains of the same cultivar have the same S-alleles and are incompatible.

Growers can consult published compatibility tables for apples online. Crab apples can also pollinate apples. Crab apples tend to have different S-alleles from commercial apple cultivars, and compatibility is, therefore, usually good.

A table for plums appeared in Fresh Quarterly issue 10.

The S-alleles for the most common pear cultivars grown in South Africa are given in Table 1.

Table 1: S-alleles for the five most common pear cultivars grown in South Africa.

Sources: Goldway M, Takasaki-Yasuda T, Sanzol J, Mota M, Zisovich A, Stern RA and Sansavini S. 2009. Renumbering the S-RNase alleles of European pears (Pyrus communis L.) and cloning the S109 RNase allele. Scientia Horticulturae 119(4) pp417–422.

The flowering of the cross-pollinator must overlap with that of the main cultivar. This seems obvious, but it’s increasingly challenging to achieve as flowering times shift under climate change and erratic weather.

Cross-pollinators should ideally start flowering just before and continue flowering for longer than the main cultivar. Growers should choose cross-pollinators with chilling requirements that match their microclimates. They can also plant two or more cross-pollinators for improved odds of overlapping flowering.

Providing enough pollen

Growers can adopt one of two strategies: plant a mix of two or more commercial cultivars or plant an orchard of a commercial cultivar and add cross-pollinators.

The advantage of mixed orchards is that pollen quantity shouldn’t be limiting. The many disadvantages are discussed later.

The number of trees added as cross-pollinators for a commercial cultivar will depend on that cultivar and the fruit type. The rule of thumb is 10% cross-pollinator trees for apple orchards. Pear and plum blossoms are less attractive to honeybees so cross-pollinators are usually planted as 15% of pear and 15%–20% of plum orchards.

Higher percentages can be used for cultivars that struggle to set.

Good cross-pollinators must flower strongly and produce plenty of pollen — profuse flowering is one reason crab apples are effective cross-pollinators of apples.

Mixed orchards typically have alternating rows of different cultivars, whereas trees planted primarily as cross-pollinators are usually positioned at intervals along each row. Bees tend to work down rather than across rows, so having cross-pollinators in the same row as the main cultivar makes sense.

The cross-pollinators don’t have to displace trees of the commercial cultivar but can instead be planted next to posts or between double rows of trees.

Remember that pollen output must keep pace with the maturing orchard. Cross-pollinators must be healthy and vigorous, or they risk becoming stunted or shaded and producing insufficient pollen for a full-bearing orchard. Cultivars prone to alternate bearing are best avoided as cross-pollinators.

Growers need to prune their cross-pollinators. Light interception and distribution are as essential for bud development and flower quality in cross-pollinators as in any other cultivar.

Managing mixed orchards

Mixing compatible cultivars with overlapping flowering periods can provide good cross-pollination while maximising the number of productive trees — it’s the most optimal solution in cherries, for example.

It’s best to combine cultivars with the same rest-breaking and thinning needs. Managing different programmes in the same orchard is not cost-effective and could lead to phytotoxicity due to spray drift.

The cultivars in mixed orchards should ideally ripen at the same time, or coordinating withholding periods and maximum residue levels becomes difficult. This can present challenges even when cultivars ripen simultaneously if the fruit is destined for different markets with different specifications.

The example of a Gala block with Granny Smith cross-pollinators illustrates some of the potential issues. One is that the optimal shading strategy for red Galas and green Grannies differ. Another is that irrigation cannot be reduced after the Galas are harvested because the Grannies are still ripening.

Eventually, picking the apples off every tenth Granny is not labour-efficient, so many growers will strip them and likely end up with mixed maturities in the bin.

Cross-pollinators are more easily managed if the fruit will not be marketed. Harry Pickstone is an excellent cross-pollinator that grows vigorously and is compatible with nearly all other plum cultivars. However, consumer tastes have passed Harry Pickstone by, and the fruit is seldom harvested for sale.

Growers should nonetheless remove the fruit of cross-pollinators — except crab apples — as part of orchard sanitation. Crab apples bear early, and the tiny fruit dry and decompose before winter.

Regardless of whether growers plant mixed orchards or a smaller number of cross-pollinators, all the trees in an orchard must be suited to the site. Cross-pollinators must be healthy and at least as tolerant of disease as the main cultivar.

When thinking about cross-pollinators, plum and pome-fruit growers could take a leaf from the livestock farmer’s book. Cross-pollinators are the rams and bulls of orchards — their performance must never be in question.